Since a relatively high compressed fuel-mixture in an engine makes a relatively perfect combustion of the mixture and provides relatively high efficiency of the engine. But in practice, when compression ratio overs 10 of a gasoline-fuel or 22 of a diesel-fuel, the fuel-mixture will burn and power to resist further travel inward of the engine piston and make not only power loses also the shock of the engine. In prior co-pending application, Ser. No. 950,434 is disclosed which is a "pressure Addible Engine" functional in increasing the pressure of compressed fuel-mixture right as the piston travels to its maximum inward position by supplying high pressure from a pressure storage chamber, which thereby causes the engine power not to resist the piston inward travel but provides higher power for piston outward travel. However, the prior invention is built in an unit engine and with little utility for retrofitting existing engines.